Why A Neurotoxin Is Closing Crab Season In California

State officials have closed both recreational and commercial fishing for Dungeness and rock crab on the California coast north of Santa Barbara to the Oregon border, due to a large algae bloom that's making the crab unsafe for consumption.

The bloom, created by an organism called Pseudo-nitzschia, produces a neurotoxin called domoic acid that can build up in marine life. It causes vomiting, diarrhea and cramping in humans — and even death, in severe cases.

California's Dungeness crabs are shipped across the U.S. and internationally, and the $60 million fishery is considered vital to the region's small fishermen. Both the commercial and recreational fishery will open as soon as test samples show the crab is safe.

So how does this toxin get into marine animals, and when might it clear out?

We spoke with Clarissa Anderson, a research scientist at the Institute of Marine Sciences at the University of California, Santa Cruz, to find out.

Algae blooms off the Pacific Coast aren't new. But why is this one so large and persistent?

This bloom has been unprecedented in its extent and its persistence. It started in May and continued on to September. And while we were expecting such a bloom in the spring and even into the summer, as has happened in years past, we did not really expect to see this continue into September.

The warm temperatures all along the North Pacific and off the West Coast are contributing to the persistence of this bloom and are allowing it to stay in the surface waters. We're now starting to see that subside a little bit as we go into fall.

Why are the waters so warm? Is it El Niño?

Well, it's true that El Niño is on its way. But we're not quite seeing the manifestations of El Niño in California just yet. For the last two years, we have seen these very high temperatures in the waters off the coast, and we're calling that the warm "blob." And that has been interacting with the bloom and sort of creating an unprecedented situation.

How long might these levels of domoic acid last in the ocean?

We know from weekly sampling that domoic acid levels, at least in the surface waters, have been coming down for the last two months. And they're pretty low now.

An algae bloom has closed crab season in California. The bloom, created by an organism called Pseudo-nitzschia,produces a neurotoxin that can build up in marine life. It causes vomiting, diarrhea, and cramping in humans and even death in severe cases.

Christy Juhasz/CDFW/Flickr

The issue with the crabs is that toxin can persist in the sediments. And those crabs are feeding on all kinds of crustaceans and shrimp along the bottom. And so the crabs are bio-accumulating it in their flesh.

So this toxin, while it might not be continuing to be produced by the algae, it certainly is in the sediments and could last there for quite some time.

Could storms or high winds clear out the algae bloom?

If we start to see some big storms from El Niño, we might start to see the water column turn over and some of these toxins flush away.

But as far as sediments go, it might be a little trickier. But I think that some storms would do a lot to clear this up.

So once a crab has this level of domoic acid in its body, how long does it take for it to clear out of the crab so it's safe to eat?

We don't have great data on the clearance rate of domoic acid from crabs. We know that they will clear it out naturally via their kidneys and they will excrete it. But we don't know just how long it will take if they continue to acquire it and feed an environment where there is a lot of domoic acid.

So the question is: Are they going to continue to acquire it, or will they be able to just excrete it over a number of days to weeks? And then we can move on and start harvesting them.

And it's not just crabs that are being affected by the neurotoxin, right? People should avoid eating fresh sardines and anchovies.

This toxin can work its way up the food web, and we've seen unusual mortality events in California sea lions and also whales farther north, in Alaska. Sea lions have been stranding within California as well as in Oregon and Washington throughout the summer and are continuing to strand.

Lauren Sommer is a science reporter for member station KQED in San Francisco. This post first ran on the KQED website.